The present invention relates to an optical head for recording signals on, and reproduce signals from, an optical recording medium such as an optical disk. The invention also relates to a recording/reproducing apparatus having an optical head of this type.
Hitherto, various types of optical recording media have been proposed as recording media for data such as video data, audio data or computer programs. Among these media are a playback optical disk, a phase-change optical disk, a magneto-optical disk, an optical card and the like. In recent years, there is the increasing demanded that the recording density and storage capacity of these optical recording media be increased.
In order to increase the recording density of such optical recording media, some measure should be taken in an optical head designed to write and read data signals on and from such an optical recording medium. It is effective to increase the numerical aperture (NA) of the objective lens or shorten the wavelength of the light source, thereby to decrease the diameter of the beam spot formed as the objective lens converges a light beam. The numerical aperture of the objective lens is 0.45 and the wavelength of the light source is 780 nm for CDs (Compact Disks) (trademark), i.e., digital optical disk which were put to practical use in the relatively early stage of optical recording. For DVDs (trademark) i.e., digital optical disks on which data is recorded at higher density on CDs, the numerical aperture of the objective lens is 0.6 and the wavelength of the light source is 650 nm.
As mentioned above, it is desired that the recording density and storage capacity of these optical recording media be increased. Therefore, it is desirable to increase the numerical aperture of the objective lens to a value greater than 0.6 and to shorten the wavelength of the light source to a value less than 650 nm.
However, various problems will arise if the numerical aperture of the objective lens is increased and the wavelength of the light source is shorted in the optical head described above. These problems are spherical aberration and chromatic aberration, both being optical problems.
Spherical aberration may occur, mainly due to a manufacturing error in the thickness of the lens or in the thickness of the transparent substrate of the optical recording medium. In the case where the objective lens is composed of a plurality of lenses, the spherical aberration may occur due to an assembling error in the gaps among the lenses. As for the error in the thickness of the transparent substrate of the optical recording medium, for example, the three-dimensional spherical aberration resulting from the error in the thickness of the transparent substrate of a disk of xe2x80x9cCDxe2x80x9d standards or a disk of xe2x80x9cDVDxe2x80x9d recording is proportional to the fourth power of the numerical aperture (NA) of the objective lens as seen from the equation [1] given below. Therefore, the influence of the thickness error becomes prominent as the numerical aperture of the objective lens increases.
W40=(t/8)xc3x97(n2xe2x88x921)/n2xc3x97NA4xe2x80x83xe2x80x83[1]
In the case of disks of the xe2x80x9cCDxe2x80x9d standards or a disk of the xe2x80x9cDVDxe2x80x9d standards, the manufacturing tolerances are of such values as to sufficiently reduce the spherical aberration resulting from the error in the thickness of the transparent substrate. In the manufacture of these disks, a specific technique is applied, setting the error in the thickness of the transparent substrate within a prescribed tolerance. It is, therefore, not necessary, in particular, to correct the spherical aberration in the optical system of the optical head. If the numerical aperture of the objective lens is further increased, however, the tolerance for the thickness error of the transparent substrate will become extremely small.
The thickness tolerance xcex94t for the substrate of, for example, the xe2x80x9cDVDxe2x80x9d standards is (0.03 mm. To maintain the same tolerance as this, the following relation expressed by the following equation [2] must be satisfied, as seen from the equation [1]:
dtxe2x89xa60.00388/NA4xe2x80x83xe2x80x83[2]
From this relation the thickness tolerance for the substrate of the disk may be obtained. The tolerance xcex94t is xc2x10.016 mm if the numerical aperture NA of the objective lens is 0.7, xc2x10.0095 mm if the numerical aperture NA of the objective lens is 0.8, and xc2x10.0074 mm if the numerical aperture NA of the objective lens is 0.85.
It is difficult, however, to raise the precision of the thickness of the disk substrate, because the error in the thickness of the substrate thickness depends on the method of forming the disk substrate. For the existing disk substrate it is difficult to reliably reduce the thickness error to 10 xcexcm or less. If the disk substrate is produced in large quantities, the yield will be low. Hence, the substrate may not be fit for mass- production.
The error in the thickness of the lenses constituting the optical system of the optical head will be considered. For molded lenses it is difficult to reliably attain such a small thickness error as 10 xcexcm or less. The thickness error of the lenses is almost equivalent to the thickness error of the transparent substrate, if it is regarded as the error in the optical path. In the case where the objective lens used has a large numerical aperture, the thickness error of the lenses will, like the thickness error of the transparent substrate, cause intolerable spherical aberration which would adversely influence the reproduced signals.
An objective lens having a large numerical aperture of 0.8 or more needs to be composed of a plurality of lenses. Even if the spacers made of synthetic resin or metal, which are interposed between the lenses, are process with high precision, it will be difficult to decrease the error in the gap between each lens and another to 10 xcexcm or less. Such an error in the gap between any two adjacent lenses of the objective lens may cause spherical aberration, like die thickness error of the transparent substrate of the optical recording medium. The gap error therefore adversely influences the signals.
In consideration of the spherical aberration occurring in the optical recording medium and in the optical head, it is virtually difficult to reduce the spherical aberration, in terms of the thickness error of the transparent substrate of the medium, to 10 xcexcm or less in the case where use is made of an optical system that comprises an objective lens having a numerical aperture of 0.8 or more and composed of a plurality of lenses.
If a semiconductor laser that exhibits a short emission wavelength is used, the problem of spherical aberration will arise, too. It is desired that not only spherical aberration, but also chromatic aberration be corrected.
The present invention has been made in consideration of the foregoing. The object of this invention is to provide an optical head comprising an objective lens composed of a plurality of lenses and having a numerical aperture of 0.8 or more, in which the spherical aberration resulting from the manufacturing errors of the optical recording medium and the lenses, and the chromatic aberration resulting from a semiconductor laser, if used as the light source of the optical head, is sufficiently decreased. Further, the invention aims to reduce the spherical aberration that occurs when the same optical system is used to light beams having different wavelengths.
Another object of this invention is to provide a recording/reproducing apparatus comprising such an optical head as described above, in which the recording density and storage capacity of an optical recording medium can be enhanced and increased.
As indicated above, it is extremely difficult to suppress the manufacturing error of the optical components so much as to reduce the spherical aberration to a negligible degree in an optical system that comprises an objective lens composed of a plurality of lenses and having a numerical aperture (NA) of 0.8 or more. It is desired that some measures be taken to cancel the spherical aberration that results from the manufacturing error of the optical components.
The spherical aberration in the optical system can be given as the sum of the aberrations occurring at the optical surfaces. Thus, the spherical aberration resulting from the manufacturing error of the optical components can be corrected by arranging optical components that cause spherical aberration of the opposite sign somewhere on the optical path extending from the light source to the optical recording medium.
The optical system of the optical head according to this invention has a group of aberration-correcting lenses. These lenses generate spherical aberration of the sign opposite to that of the spherical aberration generated at the optical surfaces in the optical system, when data signals are written on, or read from, the optical recording medium. The aberration-correcting lenses consist of two lens groups, i.e., a positive lens group a negative lens group which are spaced from each other. It is sufficient if these lens groups are respectively positive and negative in terms of power. Each lens group may consist of a single lens or a plurality of lenses.
The aberration-correcting lenses, which consist of these two lens groups, are arranged between the light source and the objective lens. The space between the lens groups is moved in the optical axis of the optical system, thus generating spherical aberration that is opposite in polarity to the spherical aberration generated in the entire optical system, with respect to the wave front of the light beam passing through the objective lens.
In the optical head, the spherical aberration is therefore cancelled at the wave front of the light beam emitted from the light source, transmitted through the objective lens and focused. As a whole, the optical system is a system in which the spherical aberration is corrected well.
Assume that the transparent substrate of the optical recording medium has a thickness error dt. Then the following equation [3] derives from the equation [1]:
dw40=(dt/8)xc3x97(n2xe2x88x921)/n3xc3x97NA4xe2x80x83xe2x80x83[3]
Hence, positive spherical aberration will occur if the transparent substrate of the optical recording medium has a large thickness error, and negative spherical aberration will occur if the transparent substrate of the optical recording medium is thin. To cancel the spherical aberration, it suffices to adjust the space between the groups of aberration-correcting lenses and generate negative spherical aberration if the transparent substrate is thick or positive spherical aberration if the transparent substrate is thin.
The spherical aberration generated by adjusting the space between the aberration-correcting lens groups, which are arranged between the light source and the objective lens, has negative polarity if the space between the lens groups is decreased or positive polarity if the space is increased. This holds true unless the light beam at the position where the aberration-correcting lens groups are provided is a converging or diverging one that can hardly be used in the optical system of an optical head.
That is, when the space between the lens groups is decreased, the light beam emitted diverges more than before, making the spherical aberration smaller than in the case the transparent substrate of the optical recording medium is thick. When the space between the lens groups is increased, the light beam emitted converges more than before, rendering the spherical aberration smaller than in the case where the transparent substrate of the optical recording medium is thin. The polarity of spherical aberration remains unchanged even if the positive lens group and the negative lens group are exchanged in position. Hence, it does not matter which lens group is arranged before the other lens group.
The actual optical system has manufacturing errors other than the thickness error of the transparent substrate of the optical recording medium. The space between the aberration-correcting lens groups is therefore adjusted to attain the best possible conditions, while the RF amplitude of the signal reproduced is being monitored.
In an optical head of this type, the larger the numerical aperture of the objective lens, the greater the spherical aberration generated by the manufacturing error of the objective lens or the manufacturing error of the optical recording medium. With the present invention, however, the spherical aberration can be reduced sufficiently even if the manufacturing error is prominent or even if the r refractive indices or absolute lengths of the various optical elements vary due to the environmental changes. This is because the groups of aberration-correcting lenses are arranged between the light source and the objective lens in the present invention.
As the means for adjusting the space between the groups of aberration-correcting lenses, thereby may be used an actuator of so-called voice-coil type, a piezoelectric actuator or the like.
A recording/reproducing apparatus according to the present invention comprises such an optical head as described above. It is designed to record data signals on an optical recording medium and to reproduce data signals therefrom.
As a special solution, the aberration-correcting lens group may function not only as means for correcting spherical aberration but also as collimator lenses, in the optical head according to this invention. In this case, the number of components can be decreased, thereby to reduce the time and labor required for manufacturing the optical head and, ultimately, to lower the manufacturing cost of the optical head. As another special solution, the aberration-correcting lens group may be arranged between a collimator lens and an objective lens. If this is the case, a beam splitter, an anamorphic prism or the like can be easily arranged, together with the aberration-correcting lens group, in the path of the light beam emitted from the collimator lens. This is because the light beam emitted from the collimator lens is a parallel beam.
An aberration-correcting lens group and an anamorphic prism may be used together in the optical head according to this invention. If so, it is desired that the aberration-correcting lens group be arranged between the anamorphic prism and the objective lens. This is because, if the anamorphic prism is arranged between the objective lens and the aberration-correcting lens group, the incident angle of the light beam to the anamorphic prism will change, inevitably causing astigmatism. In order to prevent such astigmatism from occurring, it is desirable to locate the aberration-correcting lens group at the output side of the anamorphic prism.
The optical system of the optical head according to the present invention may comprise a plurality of lenses and an objective lens having a large numerical aperture of 0.80 or more. If this is the case, the optical system is structured, effectively to correct the spherical aberration caused by that objective lens. This is because, the larger the numerical aperture and the shorter the wavelength used, the worse the spherical aberration in the optical system, and also because an objective lens composed of a plurality of lenses and possessing a large numerical aperture is likely to have large manufacturing errors.
Moreover, in the optical system of the optical head according to the invention, the light source may be a semiconductor laser having an emission wavelength of 440 mm or less and the objective lens may comprise two lens group, have a numerical aperture of 0.80 or more, have a focal distance of 1.4 mm or more and be made of vitreous material whose d line has an Abbe number of 95.0 or less. In this particular case, it is desirable that each lens group should be composed of two lenses, one being a positive lens that has an Abbe number of 55 or more and the other being a negative lens that has an Abbe number of 35 or less.
In the case where a short-wave beam is utilized, chromatic aberration may become a problem. Nonetheless, spherical aberration and chromatic aberration can be corrected altogether if the aberration-correcting lens constituting the group are made of vitreous material having the above-specified Abbe number. The structure that corrects both the spherical aberration and the chromatic aberration in the case where a short-wave beam is used will be described later.
The aberration-correcting lens group provided in the optical system of the optical head according to the invention has a considerably small numerical aperture. The spherical aberration generated by moving the lens group in the optical axis is, therefore, mainly of third order. By contrast, the spherical aberration caused by the thickness-manufacturing errors of the objective lens and transparent substrate includes high-order aberration, inevitably because the objective lens has a large numerical aperture. Consequently, the aberration-correcting lens group cannot completely cancel the spherical aberration that has resulted from the thickness-manufacturing errors of the objective lens and transparent substrate. Thus, the spherical aberration must be further corrected if the aberration-correcting lens group corrects fails to completely cancel the spherical aberration resulting from the thickness-manufacturing errors of the objective lens and transparent substrate and there remains some aberration.
In the optical system, light beams of different wavelengths may be applied, in which case it is necessary to cancel the aberration resulting from the difference in wavelength among these light beams. This aberration can be canceled by virtue of the spherical-aberration correcting effect that aberration-correcting lens group has. However, the aberration resulting from the wavelength difference may not be canceled completely because it includes high-order aberration, depending upon the type of the aberration-correcting lens group used.
In the case where the objective lens comprises two lens groups, each consisting of two or more lenses, the space between the lens groups may be changed to generate spherical aberration, in the same principle as applied to the aberration-correcting lens groups described above. Thus, the spherical aberration generated by changing the space between the aberration-correcting lens groups may be combined with the spherical aberration generated by changing the space between the lens groups constituting the objective lens. This makes it possible to correct spherical aberration of a higher order.